There is a debate about the extent to which the effect of prenatal smoking on infant health outcomes is causal. Poor outcomes could be attributable to mother characteristics, which are correlated with smoking. I examine the importance of selection on the effect of prenatal smoking by using three British cohorts where the mothers' knowledge about the harms of prenatal smoking varied substantially. I find that the effect of smoking on the probability of a low birth weight birth conditional on gestation is slightly more than twice as large in 2000 compared with 1958, implying that selection could explain as much as 50% of the current association between smoking and birth outcomes.